It helps if you keep in mind that... can (～できる）---past---> could will（～ゆく？） ---past---> would shall （～ほうがいい）---past---> should So they have a different meaning. Sometimes you can use either one you like, for example when you ask for help: "Could you help me with this?" & "Would you help me with this?" Both are correct but they have a small difference in meaning, just like "I can." &"I will." A: "If you could go back in time...would you do it?" B: "Yes! If I could, I would!" "Could" is also used for probability, like "might". (＝たぶん） "The sky looks cloudy. It could rain today." "...If I could think of more differences, I would add them." >_>